In such gantry crane apparatus for an array of racks for storage of rods contained in cassettes or storage of individual rods, one or more rails provided for movement of the gantry crane are raised considerably above the floor in order that a work station extending at right angles to the direction of travel of the gantry crane can be reached without intersecting the path of movement of the gantry train. Development of such apparatus tends to go to always higher placement of the crane travel rails in order to produce, for example, a service tunnel for machining or other treatment of the stored material within the storage location. To the same extent the application of vertical guide means near the crane columns for the load-lifting means has become of increasing importance.
In this connection it is known to provide a counterweight to affect the movement of the guiding member or members for the one or more load-lifting means, the counterweight being heavier than the guide member or members. That serves to assure that the guide member will not hang down below the crane column during horizontal travel of the gantry crane and consequently collide with the supports of the raised travel rails or with conveyor paths for entering or removing material into or out of storage.
This known form of construction has the disadvantage that a great deal of expense is necessary to adjust the movable guide member and the counterweight to each other. In addition, the load-lifting means vertically movable along the guide members have become lighter, so that particularly in the case of devices for rack storage of cassettes, the balancing of the weight of the guide members is very complicated and expensive. As a consequence of the fact that the load-lifting means are very light, it is moreover difficult to accelerate downwards the system composed of guide member, counterweight and means connecting both these parts together along with light-weight downwards moving load-lifting means.
Because of the fact that the counterweight is heavier than the guide member in question, the latter on the other hand always seeks to bump down on the load-lifting means. If now the load-lifting means settles downwards, it comes in contact with a stop at the lower end of the guiding member and then seeks to accelerate the entire system composed of the load-lifting fork, guiding member, and counterweight. It is necessary to provide limits on this effect because of the small mass of the load-lifting means.
It can happen, moreover, that the guide member hits against the load-lifting means from below when the guide member during its travel upwards is suddenly stopped. Such a shock can lead to increasing the tension of the pulling means on which the load-lifting means are suspended, as a result of which corresponding automatic safety precautions are likely to be initiated.
If for counteracting the situation just described the difference in weight between the counterweight and the guide member is made as small as possible, the values of acceleration in a system made up with these two parts will be very small for the upwards movement of the guide member, an effect which is not tolerated with the always increasing travel and operating speeds of the modern gantry cranes.
Since, as mentioned above, the travel rails for the gantry crane tend to be elevated to higher and higher locations, the available distance downward beyond the crane columns becomes greater and greater and the guiding members heavier or more stable. These greater weights are increasingly disadvantageous, corresponding to the above mentioned acceleration behavior and the overall movement behavior of guide member and its counterweight.